IS there any instance in Classical Latin of double negatives with a negative meaning? From what little I've seen, double negatives are always positive, but how is it then that in the Romance languages double negatives are negative?

As far as I know, there is no such instance...but...as far as the entire corpus of Classical Latin goes, for me, that's not saying much.

However, as to your second question, I can at least point out what happened with French, and it comes from a process of grammaticalization. Note, the following is not my own example. It comes from a lecture by John McWhorter out of UC Berkeley.

In old French, you could make a negative out of a sentence by just throwing in "ne." Like "il ne marche" (he does not walk) or "je ne sais" (I don't know).

To stengthen or emphasize the negative, the medieval French did what we often do in modern English: "I can't eat another bite," or "I can't go any farther." "Pas," aside from being part of the negative construction used in French today, also means "a step." So, when negating a sentence such as "il ne marche," and emphasizing the negative, you would say "il ne marche pas," or "he doesn't walk a step." This happened with other phrases as well..."he won't eat a bit," "she won't drink a drop," etc.

Eventually, "pas" became part of the negative construction, with the original, pure meaning sidelined and its original force weakened until there was no difference between "il ne marche" and "il ne marche pas." To quote McWhorter: "In this situation, 'pas' no longer seemed to mean 'step' at all. By the 1500s, 'pas' started to seem as if it were a new way of saying 'not' along with 'ne.' And, eventually, it was. This meant you could use it with any verb, even ones that had nothing to do with walking."

where the negatives don't cancel out, as in this translation I found online:

nothing can either furnish necessaries more richly, or present a fairer spectacle, than well-cultivated land

It seems, though, that cross-linguistically (at least for European languages), it's normal for negatives to be able to reinforce each other rather than necessarily cancel each other out. Greek (ancient and modern), English until modern times and still colloquially as mentioned, the Slavic languages are all that way. Sometimes I wonder how Latin developed such a strict rule for double negatives to be positive.

As far as I know, Modern English and Classical Latin are unique in this "Two negatives make a positive" rule, at least among Indo-European languages. It's always been my understanding that negatives just reinforced each other in English until the 18th Century prescriptivists (Samuel Johnson et. al.) decided to make English more "logical." I hadn't realized that they got their inspiration from Latin (I don't know why: the hallmark of their program was to torture English to fit into the Procrustean Bed of Latin grammar!) Does anybody know the rule in Vulgar Latin? This has all the hallmarks of the same kind of thing: the learned men who standardized "Classical" Latin remaking the rules under the influence of Greek Philosophy. Just a theory.

quendidil wrote:That point about the Old French double negative development was interesting; but what about "Je ne mange rien" forms in modern French?

It's roughly the same as cdm2003 explained with "pas." Rien is from Latin rem and originally meant "a thing" so "je ne mange rien" would have been "I don't see a thing" = "I see nothing." And eventually "rien" got reanalyzed as being negative and meaning "nothing", so you can now have a conversation like "--Qu'est-ce que tu as vu? --Rien." You can compare it to things like "je ne vois pas grand chose" where "grand chose" is roughly the same as what "rien" used to be.

Similarly with "personne" from "a person" to "no one", or "plus" from "more" to "no more," and so on. But not all these words have completely lost their positive sense, so you have the odd situation where these words can be the complete opposite in different contexts ("j'ai vu une personne" vs. "je n'ai vu personne" or less formally "j'ai vu personne"), which I've always found interesting.